1. Field of the Invention
The present invention relates to a driver for a semiconductor laser diode.
2. Related Prior Art
It is well known to control the driving current of the semiconductor laser diode (hereafter denoted as LD) to have the optical output power and the extinction ratio thereof stable by monitoring the optical output thereof with a photodiode (hereafter denoted as PD) and comparing the output from the PD with a reference. Various prior applications have disclosed such driving circuit, some of which have used a peak hold circuit to detect the peak level of the optical output from the LD and the driving current being adjusted so as to keep the peek level constant.
When the noise is superposed on the photocurrent detected by the PD and accordingly on the monitored voltage signal derived from the photocurrent, the peak hold circuit holds the peak level with the noise, which is different from the true monitored signal. Moreover, when the transmitting data continues the same level, an overshoot or undershoot may occur at the leading or falling edge of the data at the first transition after the continued level. These overshoot and undershoot may cause the error in the peak hold circuit because the peak hold circuit holds the peak level of these overshoot and undershoot. Thus, it is hard to precisely control the driving current based on the signal generated by the conventional peak hold circuit.
The present invention is to provide a LD-driver able to control the driving current of the LD with reducing the effect of the noise and the over- and under-shoots appeared in the monitored signal.